Click on Tools: Options
Scroll down to and click on Styled Text Category
Check these 3 boxes:
Show formatting toolbar - If this is on, the formatting toolbar displays by default in new message windows. If this is off, the formatting toolbar is hidden by default in new message windows. This toolbar lets you easily format text styles in new messages.
Send plain text only
Ask me each time - If this is on, you are warned when you try to send or queue a message with text styles. And you are given the option to send a plain text version, an HTML version, or both in a single message.